Photographic reversal process



United States Patent 3,210,189 PHOTOGRAPHIC REVERSAL PROCESS Ren von Wartburg, Basel, Switzerland, assignor to Ciba Limited, Basel, Switzerland, a company of Switzerland No Drawing. Filed Dec. 7, 1962, Ser. No. 242,929 Claims priority, application Switzerland, Dec. 20, 1961, 14,836/ 61 7 Claims. (Cl. 96-59) The present invention provides a novel photographic reversal developing process. Reversal development, as generally practiced, consists in exposure and development (primary development) of a silver halide emulsion to yield a negative silver image, dissolving out this silver image by treatment in an acid bath containing hexavalent chromium and reduction (secondary development) of the residual silver halide, whereupon a positive image is obtained. In this connection the words positive and negative are to be interpreted relatively. By a positive image there is to be understood a direct image of the original and a negative image in a reverse image of the original.

The afore-mentioned process has the disadvantage that very strong oxidizing agents are needed to dissolve out the primarily developed silver, such agents attack many layer supports, for example paper, thus causing the formation of a colored fog which impairs an image viewed by reflected light, and moreover, considerably affects the layer colloid. The use of a strong oxidizing agent is particularly objectionable in certain color photographic processes in which dyes or dyeforming substances are used that are destroyed by such a bath.

A process has already been proposed wherein a silver bromide emulsion is exposed and developed, the negative silver image converted into silver chloride, the silver chloride dissolved in a concentrated alkali metal chloride solution and the silver bromide reduced. However, a concentrated alkali metal chloride solution used without special additives cannot remove all the silver chloride, more especially in the case of thick photographic layers or in multi-layer materials. Even after several hours treatment with the alkali metal chloride solution the deeper layers still retain so much silver chloride that the material is fogged during the secondary development. When, on the other hand, the alkali metal chloride solution is allowed to react for a sufiiciently long time (which is uneconomical in actual practice) or when an agent stronger than an alkali metal chloride is used to dissolve out the silver salt, it is eventually possible to remove the silver chloride but at the same time a substantial proportion of silver bromide is dissolved, more especially in the upper layers of a multi-layer material.

The present invention is based on the observation that even in a multi-layer material it is possible to dissolve all the relatively readily soluble silver salt (which results from the transformation of developed silver into a salt) without simultaneously attacking the undeveloped silver halide, if the undeveloped silver halide is rendered inert towards the silver salt solvent by suitable expedients.

Accordingly, the present invention provides a process for the production of a direct silver image (i.e. of an original) in a photographic material containing at least one light-sensitive silver halide emulsion, by exposure, primary development to produce a metallic silver image, conversion of the metallic silver image into a silver salt image that is more readily soluble than the silver halide, dissolving out the silver salt image with a solvent for silver salts, and reduction of the undeveloped silver halide, wherein the material is treated, at the earliest after the 3,2l0,189 Patented Oct. 5, 1965 primary development and at the latest during the action of the silver salt solvent, with a passivating substance capable of forming with silver a compound that is even more sparingly soluble than the said silver halide. The resulting silver image can then be transformed into images of a different kind in the manner described below.

The light-sensitive emulsions contain, for example, silver bromide, preferably in a coarsely granular form. Particularly good results are achieved with silver bromide emulsions having a high content of iodide, but even silver chloride-silver bromide emulsions may be used.

The metallic silver images resulting from the primary development are advantageously transformed into silver chloride images, for example by means of a copper nitrate bath strongly acidified with hydrochloric acid, or by means of a bath containing potassium ferricynanide and sodium chloride.

The silver salt solvent or fixing bath is preferably one that is stronger than a concentrated alkali metal chloride solution; it may contain, for example, ammonia, a sulfite, a thiourea or a thiosulfate. Best results have been achieved with neutral to weakly acid baths containing sodium thiosulfate.

The substance used for passivating the undeveloped silver halide may be, for example, an iodide (such as potassium iodide, sodium iodide, ammonium iodide, or hydriodic acid) or a mercapto compound, more especially a heterocyclic mercapto compound (such as mercaptobenzimidazole or mercaptobenzthiazole) or any other compound capable of forming very sparingly soluble silver compounds. These compounds are applied before the fixing bath used for transforming the primarily developed silver into a silver salt, for example in a stop-bath following the primary development or in a separate bath following the stop-bath, and/ or they may form an ingredient of the fixing bath. Very good results are also obtained with baths containing several of these substances, or when one of these substances is contained in the stop-bath and another in the fixing bath. Good results are achieved, for example, with fixing baths containing per liter 0.01 to 0.05 g. of potassium iodide and 0.05 to 0.5 g. of mercaptobenzimidazole.

The results achieved with the present process are surprising: more especially, it could not have been expected that the above-mentioned substances would passivate the undeveloped silver halide more extensively than the silver halide resulting from the rehalogenation of developed silver images and that the solubility difference between these two types of silver salt would increase.

The passivating substances have another remarkable action. In the absence of these substances a conventional fixing bath acts more strongly upon the upper layers than upon the lower layers of a multi-layer material so that the solubility difference, for example between a silver chloride image in a lower layer and a silver bromide image in an upper layer, is even inferior to that occurring in cases where both images are located at the same depth. The treatment according to the present process has a corrective influence on this undesirable state of affairs in that the solubility of all the silver halides in the upper layers is sufficiently inhibited so that even a silver chloride image in a lower layer can be dissolved under conditions such that a silver bromide image in the topmost layer is not attacked.

The treatment according to the invention retards the fixing of the upper layers more extensively than that of the lower layers, and the fixation of a silver salt emulsion is retarded more than that of image silver transformed into silver salt. This makes it possible to achieve selective fixation of silver chloride in the presence of silver bromide even in thick layers and/ or in multi-layer materials.

Retardation of the fixation in the lower layers may also be prevented more especially by admixing with the fixing bath a fixation accelerator such as ethylenediamine, which ensures a uniform, preferred dissolution of the silver chloride throughout the layers of the material even more reliably, without thereby at the same time appreciably inhibiting the passivation of the silver bromide.

According to the present invention a light-sensitve silver bromide emulsion, for example, is rendered capable of being developed after removal of the silver chloride image, and it is of advantage to carry this out by means of the action of diffuse light, whereupon the silver bromide emulsion is reduced to metallic silver in the secondary developer. It has been found that silver bromide treated with the aforesaid passivating agents is more difficult to develop than it was prior to such treatment; this inconvenience is easy to overcome by adding a silver salt solvent to the secondary developer, thus ensuring that the passivated silver bromide, and even silver bromide containing much iodide, can be reduced without difficulty.

The requisite amount of passivating agent, the composition of the fixing bath and the duration of the treatment naturally are interdependent and depend also on the thickness of the layer and the conditions under which it is hardened so that the optimum conditions must be established experimentally in each and every case.

Silver images produced by the present process may be used as such or they may be used in conjunction with other processes, for example those in which the primary development and/or the secondary development are of the chromogen forming kind, or in which the metallic silver produced by the secondary development is used for further reactions. Accordingly, the present process is suitable for the production of gradation masks, integral color correcting masks, master prints, sound tracks on color films and the like.

The present invention is of special value for the production of opposite color photographs by the silver dye bleaching method which is based, as is known, on the destruction of certain dyes by metallic silver, resulting in a color photograph which is the opposite of the silver image. Thus, the conventional application of the silver dye bleaching method yields from a positive master a negative silver image from which, eventually, a positive color print is obtained.

Since in color photography the negative-positive process is gaining increasing ascendancy, there is considerable need for better processes for the production from negatives of positive copies, for example, images to be viewed silver image or a negative print from a negative silver image which is then used to decompose a bleachable dye; Since this process is a negative-positive one, in which the spatial distribution of the dye in the final image corresponds to the primarily developed silver, the present process provides the further great advantage in that less stringent demands on the accuracy of casting and the quality of the casting base are required than in the case of a reversal process in which the spatial distribution of the dye in the final image corresponds to the distribution of the residual halogen silver after the primary development.

Since most of the dyes suitable for use in the silver dye bleaching method are destroyed by strong oxidizing agents, such as acid bichromate baths but are not attacked, for example, by a copper nitrate bath containing hydrochloric acid, the conventional dyes are suitable for use in the present process.

It is a further special advantage of the present process that the presence of dyes capable of being bleached by silver (more especially of dyes resistant to difiustion) forbids other theoretically possible methods. Thus, many oxidizing baths that suggest themselves as alternatives to bichromate baths and which are otherwise capable of dissolving silver salts, fail in the absence of dyes resistant to diffusion. Similar reasons are probably responsible for the fact that the solubility of silver chloride in solutions of alkali metal chlorides, or the possibility of dissolving out silver salt-alkali metal chloride complexes, is considerably impaired by the presence of dyes.

Accordingly, a preferred modification of the present process consists in producing a color image which is the opposite of the master by the silver dye bleaching method by producing in a photographic material as described above (which contains at least one dye capable of bleaching by the silver dye bleaching method) a positive silver image from a negative master of vice versa, which is then transformed in known manner into a color image. In consequence the secondary development is followed by the bleaching of the dyes, for example by treatment in an acid bath containing a complex former and a dye bleaching catalyst. Finally, the silver left in the material is removed in known manner, for example by treatment in a copper nitrate bath acidified with hydrochloric acid and then in a fixing bath.

Unless otherwise indicated, parts and percentages in the following Examples are by weight:

Example 1 A double-weight photographic baryta paper is coated with a silver bromide emulsion sensitized to red, which contains the cyan dye of the formula and then with a thin layer of gelatine. The latter is covered with a silver bromide emulsion sensitized to green which contains a magenta dye of the formula This is then covered by a gelatine layer containing a yellow filter dye of the formula G r -O The top layer cast is an unsensitized emulsion contain- 19 ing a yellow dye of the formula chloride by means of a copper nitrate bath as described above, and the silver chloride then fixed in an acid fixing bath, to yield a positive print complementary to the color negative master.

Example 2 The procedure is as described in Example 1, except that no 2-mercaptobenzimidazole is added to the stop-bath.

Example 3 The procedure described in Example 1 is used, except that 0.2 g. of Z-mercaptobenzthiazole is added to the first HN-C 0 CH3 The paper coated as described above is used to make a print from a color negative in the usual manner With the aid of an enlarger.

The exposed paper is developed for 8 minutes in a developing bath containing:

Water to make 1 liter.

If desired, 0.01 g. of potassium iodide can also be added to the developer bath. After 8 minutes, development is stopped in a bath containing:

Acetic acid ml 20 Sodium acetate g 10 Z-rnercaptobenzimidazole (if desired in the form of a solution in methanol) g 0.5

Water to make 1 liter.

The material is rinsed for 3 minutes in water and the silver transformed into silver chloride in a bath containing:

Hydrochloric acid of 37% strength ml 90 Copper nitrate g 135 Citric acid g 35 Water to make 1 liter.

After rinsing in water for 5 minutes the silver chloride formed is dissolved in a bath of the under-mentioned composition without the residual silver bromide undergoing an appreciable attack.

G. Z-mercaptobenzimidazole 0.5 Sodium metabisulfite Sodium thiosulfate 50 Potassium iodide 0.05

Water to make 1 liter.

Potassium bromide g 100 Thiourea g 50 Hydrochloric acid of 37% strength m 70 2-amino-3-hydroxyphenazine g 0.001

Water to make 1 liter.

The layer is rinsed in water for 10 minutes and the excess silver remaining in the layer transformed into silver stop-bath and the same quantity to the first fixing bath instead of Z-mercaptobenzimidazole.

Example 4 The procedure is as described in Example 1, except that in addition to or in place of mercaptobenzimidazole 2 g. of potassium iodide are added to the first stop-bath.

Example 5 The procedure is as described in Example 1, except that the silver bromide emulsion used for casting is replaced by a silver bromide-silver iodide emulsion containing 96.8% of silver bromide and 3.2% of silver iodide.

Example 6 The procedure is as described in Example 4 but the base is coated with a silver bromide-silver chloride emulsion containing 96.3% of silver bromide and 3.7% of silver chloride instead of with a silver bromide emulsion.

What is claimed is:

1. In the process for the production of a direct silver image of an original in a photographic material containing at least one light sensitive silver bromide emulsion, which process comprises the steps of exposing to light the material, developing it to produce a metallic silver image, transforming the metallic silver image into a silver chloride image that is more readily soluble than the said silver bromide, dissolving out the silver chloride image with the use of a silver halide solvent, and reducing the undeveloped silver bromide, the improvement which comprises treating the material, at the earliest after the primary development and at the latest during the action of the silver halide solvent, with a passivating substance selected from the group consisting of a mercaptobenzthiazole and a mercaptobenzimidazole.

2. In the process for the production of a direct silver image of an original in a photographic material containing at least one light sensitive silver bromide emulsion, which comprises the steps of exposing to light the material, developing it to produce a metallic silver image, transforming the metallic silver image into a silver chloride image, dissolving out the silver chloride image with a bath containing a thiosulfate silver halide solvent, and reducing the undeveloped silver bromide, the improvement which comprises treating the material, at the earliest after the primary development and at the latest during the action of the silver halide solvent, with a mercaptobenzimidazole.

3. In the process for the production of a direct silver image of an original in a photographic material containing at least one light sensitive silver bromide emulsion, which comprises the steps of exposing to light the material, developing it to produce a metallic silver image, transforming the metallic silver image into a silver chloride image, dissolving out silver chloride image with a thiosulfate silver halide solvent, and reducing the undeveloped silver bromide, the improvement which comprises treating the material, at the earliest after the primary development and at the latest during the action ofthesilver halide solvent, with a mercaptobenzthiazole.

4. In the process for the production of a direct silver image of an original in a photographic material containing at least one light sensitive silver bromide emulsion, which comprises the steps of exposing to light the material, developing it to produce a metallic silver image, transforming the metallic silver image into a silver chloride image, dissolving out the silver chloride image With a bath containing a thiosulfate silver halide solvent, and reducing the undeveloped silver bromide, the improvement which comprises treating the material after the primary developing bath and before the action of the silver halide dissolving thiosulfate bath with, a bath containing a mercaptobenzimidazole.

5. In the process for the production of a direct silver image of an original in a photographic material containing at least one light sensitive silver bromide emulsion, which comprises the steps of exposing to light the material, developing it to produce a metallic silver image, transforming the metallic silver image into a silver chloride image, dissolving out the silver chloride image with a bath containing a thiosulfate silver halide solvent, and reducing the undeveloped silver bromide, the improvement which comprises treating the material after the primary developing bath and before the action of the silver halide dissolving thiosulfate bath with a bath containing a mercaptobenzthiazole.

6. In the process for the production of a direct silver image of an original in a photographic material containing at least one light sensitive silver bromide emulsion,

which comprises the steps of exposing to light the material, developing it to produce a metallic silver image, transforming the metallic silver image into a silver chloride image, dissolving out the silver chloride image with a bath containing a thiosulfate silver halide solvent, and reducing the undeveloped silver bromide, the improvement which comprises treating the material with a silver halide dissolving thiosulfate bath which contains a mercaptobenzimidazole.

7. In the process for the production of a direct silver image of an original in a photographic material containing at least one light sensitive silver bromide emulsion, which comprises the steps of exposing to light the material, developing it to produce a metallic silver image, transforming the metallic silver image into a silverchloride image, dissolving out the silver chloride image with a bath containing a thiosulfate silver halide solvent, and reducing the undeveloped silver bromide, the improvement which comprises treating the material with a silver halide dissolving thiosulfate bath which contains a mercaptobenzthiazole.

References Cited by the Examiner UNITED STATES PATENTS 1/44 Weber et al. 96-59 2,748,000 5/56 Mader 96-6O FOREIGN PATENTS 758,759 10/56 Great Britain.

NORMAN G. TORCHIN, Primary Examiner. 

1. IM THE PROCESS FOR THE PRODUCTION OF A DIRECT SILVER IMAGE OF AN ORIGINAL IN A PHOTOGRAPHIC MATERIAL CONTAINING AT LEAST ONE LIGHT SENSITIVE SILVER BROMIDE EMULSION, WHICH PROCESS COMPRISES THE STEPS OF EXPOSING TO LIGHT THE MATERIAL, DEVELOPING IT TO PRODUCE A METALLIC SILVER IMAGE, TRANSFORMING THE METALLIC SILVER IMAGE INTO A SILVER CHLORIDE IMAGE THAT IS MORE READILY SOLUBLE THAN THE SAID SILVER BROMIDE, DISSOLVING OUT THE SILVER CHLORIDE IMAGE WITH THE USE OF A SILVER HALIDE SOLVENT, AND REDUCING THE UNDEVELOPED SILVER BROMIDE, THE IMPROVEMENT WHICH COMPRISES TREATING THE MATERIAL, AT THE EARLIEST AFTER THE PRIMARY DEVELOPMENT AND AT THE LATEST DURING THE ACITON OF THE SILVER HALIDE SOLVENT, WITH A PASIVATING SUBSTNACE SELECTED FROM THE GROUP CONSISTING OF A MERCAPTOBENZTHIAZOLE AND A MERCAPTOBENZIMIDAZOLE. 